What to look for when buying memory?

Sadly this is a request from my friend. He's been bugging me about this gig stick of memory he's buying for his new system, which I believe is an Athlon 64 3400+. He started asking me questions about why the gig stick of 2100 is more expensive than a gig stick of 3200, which I wasn't able to answer. My guess is that it's a more popular amount of memory and companies are making more profit of the more used memory, but I don't know.

Second off, he started asking me about the latency, which I know lower is better because it's dealing with time... but what should you look for with timings, latency, etc?

maybe pc2100 has become an antique collector's item therefore prices have skyrocketed coz its no longer manufactured. lol

what to look for?
well the latency as you said, the best ram is one with lowest latencies that is tighter or lower timmings. also dont buy a 1gig stick, buy 2 512mb sticks instead this will save you some $$$.